A curing system using energy rays such as ultraviolet rays has become a dominant method in improving productivity and solving environmental problems of recent years. The main current photo-curing system is a free radical curing system using a (meth)acrylate-based material. However, a material of a cationic curing system using a material such as an epoxy compound, vinyl ether, and oxetane has excellent features compared to the free radical curing system, including that (a) it is excellent in surface and film curability because of being hardly susceptible to cure inhibition by oxygen; (b) it has low cure shrinkage and good adhesion to a wide range of substrates; and (c) the residual monomer amount therein can be reduced to a low level because the life time of active species thereof is long and the curing proceeds slowly even after light irradiation (dark reaction). Thus, the material has recently been examined for the application to coating materials, adhesives, sealing agents for displays, printing inks, stereolithography, silicone-based release papers, photoresists, sealants for electronic components, and the like (see, for example, Non-Patent Document 1). On the other hand, a thermal cationic curing material has a long history and is widely used in applications such as electronic components, coating materials, adhesives, and sealing materials for semiconductors (see, for example, Non-Patent Document 2).
As a material of a photocationic curing system, there is known, for example, a cyclic ether compound such as an epoxy compound, oxetane and a cyclic carbonate or a vinyl ether compound (see, for example, Non-Patent Document 3). In addition, photosensitive compositions comprising a silane compound and a photoacid generator have been proposed for the purpose of improving the abrasion resistance of cured films, and have been attempted to be used in coating materials, inks for ink jet printers, compositions for color filters, and the like (see Patent Documents 1, 2, 3 and 4). However, in each of these attempts, the step of subjecting an alkoxysilane to polycondensation to provide a polyalkoxysilane is provided before the step of obtaining a desired photosensitive composition. Thus, a solvent is positively mixed to obtain the desired photosensitive composition in a state containing the solvent used during the synthesis or to dissolve the polyalkoxysilane. For that reason, the resulting photosensitive composition has a problem with curability. In addition, the composition is insufficient in adhesion to a substrate because the resultant cured film is rigid.
A photosensitive composition has also been proposed in which there are blended a polyfunctional (meth)acrylic ester and a compound in which (meth)acryloyl groups are introduced into a polyorganosilicate (see Patent Document 5). The photosensitive composition has a problem with curability and adhesion to a substrate because it can be free radically cured by the main use of the (meth)acryloyl group. In addition, a non-solvent photosensitive composition containing a non-solvent silane compound has been proposed (see Patent Document 6). In the proposed composition, a cationic reactive compound and a cationic polymerization initiator are blended with a tetraalkoxysilane; an epoxy compound is further blended therein to increase the adhesion thereof to substrates. However, the composition is insufficient in curability and in cleaner resistance and water resistance of a cured film thereof.
Further, for the purpose of improving water resistance, an ultraviolet curing type resin composition has been proposed which comprises a phenol compound, a compound having a carbon-carbon unsaturated bond, and a photoacid generator (see Patent Document 7). However, the proposed ultraviolet curing type resin composition has high viscosity and provides a cured film insufficient in adhesion to a substrate and cleaner resistance. An increased blending amount of the compound having a carbon-carbon unsaturated bond for reducing the viscosity does not result in improved physical properties of the cured film. A photocationic curable composition has also been proposed which uses a vinyl ether compound in which vinyl ether groups are preliminarily introduced into a phenol (see Patent Document 8). According to this proposition, a haloalkyleneoxy vinyl compound represented by general formula (2) below is reacted with a particular phenol novolak resin in the presence of a basic compound to provide a phenol novolak-type vinyl ether compound, in which another vinyl ether compound and a photoacid generator are further blended to offer a cationic curable composition.X—{(R1)p—O—}q—CH═CH2  (2)(wherein X represents a halogen atom; R1 represents a straight-chain or branched alkylene group having a carbon number of 1 to 10; p represents an integer of 1 to 10; and q represents an integer of 0 to 10.)
However, the curable composition is high in viscosity because of using the phenol novolak-type vinyl ether compound. A larger blending amount of the low molecular weight vinyl ether compound for reducing the viscosity results in insufficient adhesion to a substrate.
In addition, Patent Document 9 discloses a curable composition in which a polyfunctional alicyclic epoxy compound, a phenol compound having a particular structure, and a photocationic initiator are mixed together. However, the curable composition disclosed in Patent Document 9 is gradually thickened during storage and therefore is required to be further improved in long-term storage stability.
Patent Document 1: Japanese Patent Laid-Open No. 2000-001648
Patent Document 2: Japanese Patent Laid-Open No. 60-186570
Patent Document 3: Japanese Patent Laid-Open No. 2000-327980
Patent Document 4: Japanese Patent Laid-Open No. 10-104414
Patent Document 5: Japanese Patent Laid-Open No. 10-087765
Patent Document 6: Japanese Patent Laid-Open No. 2001-348515
Patent Document 7: Japanese Patent Laid-Open No. 10-330463
Patent Document 8: Japanese Patent Laid-Open No. 11-349516
Patent Document 9: WO2005/019299
Non-Patent Document 1: Masahiro Kadooka et al., “Kachion Koka Gijutsu No Kogyo Tenkai (Industrial development of a cationic curing technique)”, MATERIAL STAGE, vol. 2, No. 2: page 39 to 92, issued by Technical Information Institute Co., Ltd., May 10, 2002
Non-Patent Document 2: Review “Epokishi Jushi (Epoxy resin)” ed. Epokishi Gijutsu Kyokai (society of epoxy techniques), vol. 3 & vol. 4
Non-Patent Document 3: Yasuyuki Tanimoto, “Fotoporima Hyomen Kako Zairyo (A material for processing the surface of photopolymers)”, vol. 50: page 51, issued by Bun-shin Publishing Co., Ltd.